The invention relates to power trains, especially to power trains which can be installed between the engines and the wheels of motor vehicles. More particularly, the invention relates to improvements in power trains which operate with infinitely or continuously variable transmissions of the type employing two adjustable sheaves and an endless flexible torque transmitting element which is trained over the sheaves. Transmissions of the just outlined character are described and shown, for example, in the aforementioned copending patent application Ser. No. 07/778,462, in commonly owned U.S. Pat. No. 5,046,991 granted Sept. 10, 1991 to Friedmann for "Continuously Variable Speed Transmission" and in commonly owned copending patent application Ser. No. 07/616,371 filed Nov. 21, 1990 by Friedmann for "Continuously Variable Transmission". Still more particularly, the invention relates to improvements in power trains of the type wherein a continuously variable transmission can drive one or more pairs of wheels in a motor vehicle by way of one or more engageable and disengageable clutches which are connected in series with one sheave of the transmission.
Power trains of the above outlined character are disclosed, for example, in German Pat. No. 28 28 347 granted Oct. 21, 1982 to Rattunde and in German patent application No. 35 38 884 (published May 21, 1987) of Svab. The power trains of Rattunde and Svab employ a torque sensor which is installed between the engine of a motor vehicle and the input member of the transmission and constitutes a torque-responsive valve. The valve receives pressurized fluid from a pump. The arrangement is such that the valve is closed, at least in part, in response to abrupt rises of torque in order to effect an appropriate increase of fluid pressure in the motors (such as cylinder and piston units) which are employed to move the axially movable flanges of the sheaves relative to the axially fixed flanges. This results in more pronounced engagement between the flanges of the sheaves and the adjacent portions of the endless torque transmitting element which is trained over the sheaves. The valve which constitutes or forms part of the torque sensor comprises two discs having confronting ramps for rolling elements which are installed between the discs. The discs are biased toward each other, and hence against the rolling elements between them, by fluid pressure which is generated by the pump. When the torque which is transmitted from the engine to the input member of the transmission by way of the valve undergoes a pronounced increase, one of the discs turns relative to the other disc whereby the rolling elements move the discs apart and the axially movable disc alters the rate of flow of pressurized fluid from the valve. This entails a rise of fluid pressure and an increase of the force which the flanges of the sheaves exert against the adjacent portions of the torque transmitting element. The discs of the valve which forms part of or constitutes the torque sensor not only regulate the pressure of fluid as a function of the magnitude of transmitted torque but they also serve as a means for transmitting torque from the engine to the transmission. The pump must be designed to supply fluid at a very high pressure because the two discs and the rolling elements of the valve (which receives pressurized fluid from the pump) must transmit torque from the engine to the input member of the transmission. A considerable amount of pressurized fluid is permitted to escape from the valve so that losses due to leakage of pressurized fluid are quite high.
The same pump is used to adjust the ratio of the infinitely variable transmission. To this end, the power trains of Rattunde and Svab employ a regulating valve (e.g., a complex slide valve) which controls the pressure in the cylinder and piston units for the axially movable flanges of the sheaves. Thus, the regulating valve can raise the pressure in one of the units while reducing the pressure in the other unit to thus ensure a radial displacement of the torque transmitting element and hence a change of the transmission ratio.
A drawback of the just described conventional power trains is that the pump must deliver fluid at an elevated pressure several times the pressure which is required for changing the transmission ratio. This is due to the fact that the same pump also serves to raise the pressure of fluid in response to changes of transmitted torque. At the same time, the pump must convey the fluid at a high rate in order to ensure that the transmission ratio can be changed very rapidly in spite of the elevated pressure of conveyed fluid. Otherwise stated, losses of pressurized fluid are very high and continuous. Such losses are proportional to the rate of fluid flow at the aforediscussed elevated pressures. In addition, the conventional power trains employ a number of complex and expensive flow controlling and other regulating components which are prone to malfunction.